Candela(cd) is the unit of measurement for luminous intensity (I), for the luminous flux emitted by a light source. By definition, it is the energy emitted in one second by a black body or Planckian radiator with 1/60 cm2 surface area, at the melting temperature of platinum, at 1,770°C. In contrast to luminous intensity, luminance is expressed in candela per unit area (cd/m2).
The reference quantity between luminous intensity and luminous flux, whose unit is the lumen( lm), is the solid angle or beam angle, specified in steradian. The luminous intensity in candela results from the ratio of the luminous fl ux in lumen to the solid angle. The distribution of luminous intensity is determined by the design of the luminaire with its built-in reflectors and lenses. The luminous intensity distribution of lighting fixtures depends on the solid angle and is represented as a luminous intensity distribution curve (LVK) in a polar coordinate system for all solid angles.
Graphical representation of the light distribution curve (LVK) in the polar coordinate system, graphic: licht.de
In the case of light-emitting diodes, the dependence of the luminous intensity on the beam angle is of particular interest. There is a negative relationship between them, which is shown by the fact that the perceived luminous intensity decreases with larger beam angles and increases with smaller beam angles. A light emitting diode with a narrow beam angle is perceived by the observer as more intense because the light energy is concentrated on a smaller area.